The present application pertains to plasma treatment chambers such as those used in semiconductor integrated circuit fabrication.
FIG. 1 shows a plasma chamber which may, for example, be used in the fabrication of semiconductor integrated circuits. As shown, a wafer W (e.g., on which one or more semiconductor integrated circuits are formed) is positioned between first and second electrodes e1 and e2 located at opposite sides of the chamber. The wafer W is also located between north m1 and south m2 poles of a magnet also on opposite sides of the chamber, which sides are orthogonal to the sides at which the electrodes e1 and e2 are located. A low pressure gas G is introduced into the plasma chamber through an inlet port, such as a shower head S. A voltage source V applies an oscillating voltage (of, for example, 13.58 MHz) across the electrodes e1 and e2 to produce an electric field E directed between the two electrodes e1 and e2. This tends to cause the molecules of the low pressure gas G to gyrate in a cycloid motion. The north and south poles m1 and m2 of the magnet introduce a magnetic field B directed between the two poles, which magnetic field B is orthogonal to the electric field E. This tends to increase the collisions of the gyrating molecules thereby completely ionizing them to form the plasma P over the wafer W. A coolant C, such as liquid He, may be circulated on the underside of the wafer W to cool it during treatment.
FIG. 2 shows a more detailed view of certain parts of an actual plasma chamber 100, such as the MXP Centura(trademark), distributed by Applied Materials, Inc.(trademark), located in Santa Clara, Calif. The chamber 100 has cylindrically shaped sidewalls 105. A cathode 110 is located at the bottom of the chamber 100. A pedestal 120 is secured to the cathode 110. (Actually, additional parts may be secured to the cathode 110 between the cathode 110 and the pedestal 120, such as an O-ring and aluminum sheet interface. These are omitted for sake of brevity.) The pedestal 120 is secured by screwing screws through the holes 122 of the pedestal 120 and the holes 112 of the cathode 110. A quartz pedestal liner ring, not shown, may then be placed in the chamber 100 surrounding the pedestal 120 (for purposes of improving the uniformity of the flow of the plasma gas over the entire wafer W). A transparent quartz cover or focus ring 150 may then be secured to the top of the chamber 100 to form a gas-tight seal, thereby confining the plasma P within the chamber 100 and isolating the wafer W from external contamination. As shown, the quartz cover or focus ring 150 is secured by screwing screws 130 through holes 132 to the chamber 100 or another part secured therein (not shown for sake of brevity). A quartz cap 140 may be placed on top of each screw 130.
The wafer W may be secured to the pedestal 120 in one of two ways. The pedestal 120 can be an electrostatic chucking pedestal. Such a pedestal 120 can generate an electrostatic charge that holds the wafer W in place during treatment Alternatively, an ordinary pedestal 120 may be used. In such a case, the wafer W is then clamped to the pedestal 120 using a clamping ring 160. As shown, the clamping ring 160 has plural tips 170 which extend radially towards the interior of the ring 160. The dimensions of the clamping ring 160 are such that the ring 165 thereof has a greater diameter than the wafer W and does not touch the wafer. Rather, only the tips 170 contact and touch the wafer W. The tips 170 have holes 172 to enable screwing the clamping ring 160 to the pedestal 120 using (e.g., metal) screws 131 (which in turn are covered by graphite plugs, not shown) so that the tips 170 contact and press down on the wafer W, thereby holding it in place.
Plasma treatment is commonly used to etch structures on the wafer, such as polycrystalline silicon (poly) and oxide structures. Specifically, wafer structures not to be etched are typically covered with a mask whereas wafer structures to be etched are left exposed. The treatment using the plasma erodes the exposed structures.
Such a plasma erosive effect is also incurred by the various parts within the chamber 100. This reduces the life time of the parts. Moreover, because such pats are eroded while treating the wafer, the eroded material of the parts is introduced in the plasma chamber 100 as a contaminant. This tends to reduce the yield of the semiconductor integrated circuits formed from the treated wafers. Two parts specifically subject to the plasma erosive effect are the screws 130, used to secure the quartz focus ring or cover 150 (and, theoretically, can be used to secure other objects within the plasma chamber 100), and the clamping rings 160.
FIG. 3 shows the screw 130 and cap 140 assembly in greater detail. The screw 130 includes a threaded shaft 135 and a head 137 affixed to, and integral with, the top of the screw 130. The screw 130 is preferably made of a polyimide material, such as the material marketed under the brand name Vespel(trademark) by DuPont Engineering Polymers,(trademark) located in Newark, Del. The screw head 137 has a concave shape. Specifically, the screw head 137 has a recess or slot 139 formed therein for receiving a screw driver blade. As such, the screw head 137 has sharp xe2x80x9cpointedxe2x80x9d edges 131 and 133, which edges 131 and 133 each have a small surface area.
The screw 130, in particular, the screw head 137, is subject to erosion by the plasma. (The shaft 135 is typically completely screwed into another object within the chamber 100 such as the hole 132. Thus, only the screw head 137 is exposed to the plasma of the chamber 100.) In an effort to extend the lifetime of the screw 130, a protective quartz cap 140 is typically placed over the screw. The quartz cap 140 has an opening 141 which is dimensioned larger than the screw head 137 so that it can be placed over, and can cover, the screw head 137.
There are several problems with the screw 130 and quartz cap 140 assembly. First, it is difficult to make a cap 140 that fits tightly on the screw 130. This has two consequences. Specifically, some plasma is able to reach the screw 130 and erode it. The eroded material produces a build up of contaminating particles within the opening 140. This contaminates the wafer. In addition, the useful life of the screw 130 is limited to only about 100 hours before it is too badly eroded to be reused.
Second, during use, the vibration of the chamber 100 can dislodge the caps 140 causing one or more to be damaged or lost under (or within) one of the many removable parts of the machine (only some of which are shown in FIG. 1). As the quartz caps 140 are quite expensive (e.g., around U.S. $40 each), this substantially increases the cost of semiconductor integrated circuit manufacture.
In addition, the chamber 100 is utilized in an application in which contamination is controlled. The operator of the chamber must therefore wear protective gloves while inserting and screwing in the screws 130. As this requires both placement of the screw in a hole and use of a screw driver, the operation requires a large amount of time, thereby reducing the amount of time that the chamber 100 can be utilized in the fabrication process.
It is an object of the present invention to overcome the disadvantages of the prior art.
This and other objects are achieved by the present invention. According to one embodiment, an apparatus is provided for treating a wafer under fabrication with an erosive plasma, in a contamination controlled environment. The apparatus includes a chamber for containing the wafer to be treated by the plasma, and for isolating the wafer from contaminants external to the chamber during treatment. The chamber also includes one or more plasma erosion resistive screws. Each screw has a shaft secured within the chamber so that the shaft is unexposed to the plasma, and a raised head which is integral with, and made of the same material as, the shaft. The head has a continuous, surface shape with a reduced number of edges so as to reduce the accumulation of charge thereon, thereby resisting erosion by the plasma.